1. Field of the Invention
This invention is concerned with the preparation of alkyl carboxylates mixtures which are enriched in alpha-methylalkyl carboxylates. Such mixtures are prepared by reaction of carboxylic acids with olefinic compounds in the presence of a particular type of crystalline zeolite catalyst.
2. Description of the Prior Art
The addition of carboxylic acids to olefins to make esters is known. The chemical literature describes the use of Lewis acid catalysts, such as BF.sub.3, to promote the reaction. However, particularly in the case of internal olefins, the reaction will result in addition of the carboxylic acid to both ends of the double bond, thereby giving rise to a mixture of carboxylate products. Mineral acids (e.g., H.sub.2 SO.sub.4) are also reported to catalyze the reaction, but the result is much the same, i.e., non-selective addition of the carboxylic acid to either side of the carbon-carbon double bond. Trevillyan, U.S. Pat. No. 3,492,341; issued Jan. 27, 1970 discloses preparation of alkyl carboxylate esters by reacting carboxylic acids with 1- or 2-monoolefins over a mordenite zeolite catalyst.
In the past, the only known reaction route to directly produce alkyl carboxylate mixtures enriched in alpha-methylalkyl carboxylates has required the utilization of expensive alpha-olefins. Reaction with internal or mixed olefins using common acid catalysts has necessitated physical separation of the isomeric variants of the alkyl carboxylate product by other means, such as distillation, in order to isolate a product which is rich in the .alpha.-methylalkyl carboxylate.
Alkyl esters of carboxylic acids are useful as solvents, plasticizers and chemical intermediates. Alpha-methylalkyl carboxylates are particularly useful for making secondary alcohols with hydroxyl attachment at the 2-carbon. By utilization of the herein disclosed method, alpha-methylalkyl carboxylate enriched products normally derived from pure alpha-olefins can now be prepared from less expensive linear olefin mixtures.